1. Field of the Invention
The invention relates generally to polymers and the use thereof to assist in aggregating mineral components in aqueous mineral slurries to release and separate individual components of the slurry, which may then be recovered from the slurry.
2. Related Technology
Many industrial processes involve the dispersion of minerals in water to assist in the separation and recovery of mineral or other components. The mining industry is the predominant user of such processes, wherein mineral ores are ground and slurried in water to allow separation and recovery of desired components. The residual mineral components in the slurry, referred to as gangue or tailings, are then often deposited in pits or ponds, often called tailings ponds, where solids are expected to settle to allow recovery of the supernatant water, and ultimate consolidation of the remaining mineral solids. Coal, copper, and gold mining are but a few of the mining processes that employ this technology.
The slow rate of mineral solids settling in tailings ponds is often a serious economic and environmental problem in mining operations. If an objective of such processes is to recover water for reuse or disposal, lengthy pond residence times, often measured in years, can cripple process economics. Further, huge volumes of ponded slurry can be environmentally and physically dangerous. Dike failures of coal slurry ponds in the United States attest to both these dangers.
If the ponded slurry is predominantly composed of coarse minerals, the settling rate in tailings ponds is not generally an environmental or economic problem. In this instance, solids settle quickly and consolidate to disposable consistencies, and water is easily recovered. But when components of the ponded slurry are very fine materials, settling is often hindered and, in some instances, may take years to occur.
A major undesired component of many mineral slurries is often clay. Clays have a variety of chemical compositions but a key difference in how a clay behaves in a mineral slurry is whether it is predominantly in a monovalent (usually sodium) form or in a multivalent (usually calcium) form. The effects of the varying chemical compositions of clays are well known to those in industry. Monovalent clays tend to be water-swelling and dispersive, multivalent clays generally are not.
Water-swelling and dispersive clays cause many of the problems in mineral processing and tailings dewatering. These clays tend to be monovalent, sodium clays, such as bentonite, which is largely composed of montmorillonite. These clays can be expressed as Na.Al2SO3.4SiO2.H2O.
Further, if the clays are very finely divided, the problem is often magnified. If the clay particles are easily broken down to even finer particles through shearing in processing, problems can be compounded. Layered, platelet, or shale-like forms of clay are particularly sensitive to mechanical breakdown to even finer particles during processing.
In mineral processing, additives are often used to facilitate removal of specific components. Frothers used to separate and float ground coal particles are an example of this. In this instance, the desired component to be recovered is an organic material such as coal, but similar processes are used for mineral recoveries. In almost all mining processes the remaining slurry must be separated to recover water and consolidated solids.
Since the late 1960s, a new mining industry has been operating in the northeast of the Canadian province of Alberta. The deposits being mined are referred to as the Athabaska oil sands. The deposits are formed from a heavy hydrocarbon oil (called bitumen), sand, clay, and water. In processing the deposit, the ore is slurried in warm or hot water with the objective of separating the bitumen from the sand and clay, recovering the bitumen by flotation, recovering the water for reuse, and disposing of the dewatered residual mineral solids in site reclamation. The oil sand deposits contain the second largest quantity of oil in the world, second only to Saudi Arabia's. Consequently, separation, water recovery, and solids disposal are carried out on an industrial scale never before seen.
The first objective in oil sands processing is to maximize bitumen recovery. Slurrying in warm or hot water tends to release bitumen from the minerals in the ore, in a pipeline process called hydro-transport, while the slurry is transported via pipeline to a primary separation unit. Various chemical additives, including caustic soda or sodium citrate, have been used to improve dispersion of the ore's components into the process water and to accelerate separation of the bitumen from the sand and clay for greater bitumen recovery. In the hydro-transport process, sand is relatively easily stripped of bitumen and readily drops out and is removed through the bottom of the primary separation unit; the clays are the principal problem. Clays, associated with divalent or other multivalent cations, particularly calcium and magnesium, contributed by, for example, process waters are recognized to deter efficient separation and flotation of the bitumen. The use of additives such as caustic soda or sodium citrate aid in the dispersion to inhibit clay's deleterious effects. Sodium citrate is a known dispersant and also acts as a water-softening agent, to sequester calcium and magnesium ions.
While improving recovery, these additives often have residual negative effects following bitumen separation by inhibiting subsequent water removal from the clay. A great deal of research has gone into studying the various types of clays found in the oil sands deposits. Different clays affect bitumen separation differently, often in ways not completely understood, and differences in the clays affect the clays' subsequent separation from the process water. Since ore is a natural deposit, the separation process is at the mercy of clay type and content, and the level of divalent ions. Pump and pipeline shear acting on the slurry break down clay into finer clay particles to further negatively affect the separation process. Various ore sources are often blended prior to hydro-transport in an attempt to mitigate the effects of clays. Compressed air may be introduced into the hydro-transport pipeline. The air dissolves under pressure and, as pressure is released ahead of the primary separation vessel, bubbles form to help float the bitumen.
In the separation process, the floated bitumen overflows to further processing. Typically, the sand and any coarse clays settle quickly into the base of a conical primary separation unit. The withdrawal rate of this coarse segment can be controlled. The largest volumetric component, called middlings, is the middle stratum above the coarse layer and below the bitumen float. The middlings comprise a dispersion of the fine clays. The industry considers these fine clays to be any size less than 44 microns in diameter. These clays usually form a very stable dispersion. Any dispersive additives further increase the stability of the clay slurry. If the dispersant, or any other additive, increases middlings viscosity in the primary separation unit, then bitumen flotation and recovery may be hindered.
In existing processes, the conditions that promote efficient dispersion and bitumen recovery appear to be diametrically opposed to the conditions that subsequently promote downstream fine clay separation, solids consolidation, and water recovery. The longer it takes to recover and reuse the process water, the more heat and evaporative losses occur. The tradeoff between efficient bitumen extraction and downstream disposal of mineral solids is an expensive problem for the oil sands industry.
In the extraction process, middlings are continuously withdrawn from the center of the primary separation unit. Both the heavy, easily settled sand/coarse clay component, withdrawn from the conical bottom of the primary separation unit, and the middlings component are usually subjected to additional cleaning and mechanical dewatering steps to recover any bitumen that is not floated off in the primary separation unit. The middlings may be hydrocycloned to increase density. The middlings then generally report to a thickener, where high molecular weight sodium/potassium/ammonium-acrylate/acrylamide-based copolymers (called flocculants) are added to coagulate and flocculate the dispersed middlings' fine clays. Four to five hours of residence time are generally required in the thickener to produce a thickened underflow (to begin to increase clay solids for use in final solids consolidation) and to produce clarified overflow water for reuse in the process. Thickeners are immense, expensive mechanical separators with massive holding volumes.
The final objective of the oil sands process is to produce dense, trafficable solids for site reclamation and to recover water for process use. The two mineral process streams, sand/coarse clay from the primary separation unit, and middlings (often thickened as described above) are either pumped to separate containment areas (called ponds) or are combined and then sent to ponds. Both approaches have created problems, with which the industry is grappling. The combined streams (called combined tailings, or CT) have produced a condition wherein the coarse sand and clays have settled relatively quickly in the ponds, but the fine clays have not. Instead of the desired settling and recovery of supernatant water, the upper layer in these ponds forms an almost permanent layer of suspended fine clays, referred to as mature fine tails (MFT). The clay content in this relatively fluid, almost permanent layer of MFT generally ranges from 40 wt % to 50 wt % solids. When the middlings are pumped separately to ponds, the same condition is immediately created. The existence and size of these ponds threaten the very future of the industry. Government has ordered that these ponds of MFT must be re-processed, water recovered for reuse, and dewatered solids consolidated to restore the mined sites.
The oil sands industry has made a concerted effort to reprocess the MFT into what are called non-segregating tailings (NST). By this is meant sand and clay tailings of varying particle sizes that, when pumped to ponds, do not segregate by particle size upon settling but, rather, settle in a non-segregating manner, more quickly releasing supernatant and/or underflow drainage waters, and ultimately producing a trafficable solid that can be used for mine site restoration. Heat is still lost after the NST slurry is pumped to ponds and the warm water still evaporates. Any method or procedure that could recover more warm water within the operating process, and that could produce easily-dewatered, non-segregating tailings immediately after the separation process, would be of great benefit to the oil sands industry.
In Nagan U.S. Pat. No. 6,190,561 and its counterpart Canadian Patent No. 2,290,473, the entire respective disclosures of which are incorporated herein by reference, Nagan describes a process using “zeolite crystalloid coagulants (ZCC)” as a method of water clarification. This sodium or potassium zeolite, referred to in the patent as ZCC, is used in a specific sequence to coagulate solid particles and separate them from an aqueous dispersion. The specified sequence comprises, first, providing an aqueous suspension of particulate matter containing (and maintaining) multivalent cations (and optionally adding additional multivalent cations, such as cationic polyacrylamide), then adding a zeolite crystalloid coagulant in sufficient amount to effect coagulation of the particulate matter by ion exchange between said adsorbed cations and the sodium or potassium present in the ZCC. This specific sequence is very effective in coagulating the cationic solids.
In the '561 and '473 patents, Nagan describes the procedure for producing this type A zeolite by reacting sodium aluminate and either sodium or potassium silicate, relatively inexpensive and commercially available chemicals. Both sodium silicate and sodium aluminate are available as bulk liquids.